Forget about MSN/Yahoo.
Forget about Skype.
Forget any of those dull money-hungry low quality SIP VoIP services.

USE WENGO INSTEAD!

http://openwengo.org

WengoPhone is a communication tool based on the SIP protocol, developed by the OpenWengo community and released under the GNU General Public License. It is more than just VoIP software: it is a full-fledged communications tool that lets you enjoy free calls, video conferencing, SMS, chat over many existing protocols (MSN, AIM/ICQ, Yahoo!, Jabber).

I think nuff is said. Wait, no?

Ok, there are other features like:
-videophoning under linux too!
-sms sending
-FREE CALLS TO LANDPHONES TO 26 countries worldwide!
EDIT: forgot about FireFox extension :F

I will change. And you?

I'm interested in a few of these programs. I am a bit dissappointed with Skype under Linux at the moment.

Wengo is SIP based so lots of other clients can comunicate. It also uses Qt so will integrate nicely under KDE rather than being a hideous abomination like anything Mozilla can be.

Edit: Installed. The English translation isn't that great: "Your are authentified on the Wengo platform"